The present invention relates generally to wind turbines, and particularly to techniques for installing and anchoring foundations for offshore wind turbine systems. Advantageously, certain embodiments of the present technique may be employed to increase the water depth at which offshore wind turbines may be located.
Wind turbines are generally regarded as an environmentally safe and a desirable source of renewable energy. In summary, a wind turbine harnesses the kinetic energy of wind and transforms this kinetic energy into electrical energy. Thus, electrical power can be generated with virtually zero emissions, unlike existing natural gas-fired or coal-fired power generation technologies. To maximize the efficacy of power generation and to simplify connection to a power grid, several wind turbines are often located in proximity to one another in what are generally referred to in the pertinent art as “wind farms.” Advantageously, these wind farms are located in regions having relatively strong winds, such as, for example, at offshore locations.
At offshore locations, in order to better access the prevailing winds around the year and to limit visibility from the shore, it is desirable to install wind farms at increasing distances from the shore, and consequently deeper water depths. Generally, for shallow water depths (for example, less than 20 meters), the typical foundation structure for an offshore wind turbine installation comprises a monopile. A monopile is essentially a long cylindrical caisson, assembled in sections on-shore and driven to the required soil penetration depth at the offshore installation site. However, for increasing water depths, a simple, traditional monopile foundation may prove uneconomical due to the correspondingly larger pile diameters and thicknesses that are required.
The overall structural design of the foundation structure of an offshore wind turbine is determined by the dynamic wind and wave loads under various turbine operational modes and by extreme loads imposed during high wind speed conditions and stormy sea states (during which the turbine is typically not operational). To minimize fatigue loading, the stiffness of the foundation structure should be desirably sized such that the overall natural frequency of the wind turbine/foundation system is outside the frequency range of excitation due to the rotor operation and the hydrodynamic wave loading. A typical industry practice is to size the stiffness of the foundation structure such that the overall system natural frequency is higher than the excitation from the rotor revolution but lower than the excitation from the blade passing the wind turbine tower. With this target natural frequency, the monopile diameters and thicknesses reach their current manufacturing limits for water depths between 20-25 meters (depending on the site conditions). At present, manufacturing capability is limited to fabricating monopiles of approximately 50 millimeters in thickness and 5.5 meters in diameter. Even if larger piles were to become available, the feasibility of such foundations is limited due to the limited availability and increasing expense of larger offshore pile driving hammers.
In the past, installation water depths greater than 20 meters have been contemplated by incorporating a tripod-style foundation structure. A typical tripod-style structure comprises, for example, 3-4 lateral braces that reinforce a central tubular column. These braces typically terminate in grouted sleeves, through which small diameter piles are driven to anchor the foundation and transmit the brace loads to the soil. As the braces of a tripod-style structure are rigidly attached to the central tower section, they have to be carefully designed to limit the stress concentration at these locations. In the past, the fatigue design of tripod joints and other welded connections in such structures have presented difficulty in design. In addition, the varying soil conditions and water depths in an offshore wind farm could result in varying brace diameters and joint geometries. In summary, the optimal tripod foundation structure is site specific and must be redesigned at each implementation. As a result, mass production of such tripod type structures may prove uneconomical.
Therefore, there is a need to design a foundation structure that will provide the necessary stiffness fatigue resistance and ease of geometric scalability at increased water depths. There is also a need for a more robust design of a foundation structure that is less sensitive to changes in water depth and underlying soil conditions.